This application claims priority from EP application no. 03252585.9 filed Apr. 24, 2003, the contents of which is incorporated herein in its entirety.
1. Field of the Invention
The present invention relates generally to a lithographic apparatus and more particularly to a method of exposure using multiple exposures.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. The apparatus generally comprises a radiation system for supplying a beam of radiation, a support structure for supporting a patterning device, the patterning device serving to pattern the beam, a substrate table for holding a substrate, and a projection system for projecting the patterned beam of radiation onto a target portion of the substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g., comprising part of, one or several dies) on a substrate (e.g., a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
The term “projection system” used herein should be broadly interpreted as encompassing various types of projection system, including refractive optical systems, reflective optical systems, and catadioptric optical systems, as appropriate for example for the exposure radiation being used, or for other factors such as the use of an immersion fluid or the use of a vacuum. Any use of the term “lens” herein may be considered as synonymous with the more general term “projection system.” The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components. The radiation system as well as the projection system generally comprise components for directing, shaping or controlling the projection beam of radiation. Generally, the projection system comprises means to set the numerical aperture (commonly referred to as the “NA”) of the projection system. For example, an adjustable NA-diaphragm can be present in a pupil of the projection system. The radiation system typically comprises adjusting means for setting the outer and/or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution upstream of the mask (in a pupil of the radiation system).
The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more mask tables). In such “multiple stage” machines the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposure. The lithographic apparatus may also be of a type wherein the substrate is immersed in a liquid having a relatively high refractive index, e.g., water, so as to fill a space between the final element of the projection system and the substrate. Immersion liquids may also be applied to other spaces in the lithographic apparatus, for example, between the mask and the first element of the projection system. Immersion techniques are well known in the art for increasing the numerical aperture of projection systems.
A circuit pattern corresponding to an individual layer of an IC device generally comprises a plurality of device patterns and interconnecting lines. Device patterns may comprise features of different spatial arrangement such as, for example, line-space patterns (“bar patterns”), capacitor contact patterns, patterns of contact holes and DRAM isolation patterns. A feature is not necessarily characterized by a shape whose line elements define a closed contour. For example, a spatial arrangement of extremities of two neighboring features and a space between the two extremities may also, in the context of the present text and claims, be referred to as a feature.
In the context of the present text and claims, sizes of features are referred to as those sizes that the features nominally have at substrate level. At a mask, the size of a feature is M times larger than the nominal size, where M is the magnification of the projection system (typically, |M|=¼ or ⅕). Generally, additional size deviations at the mask are introduced to compensate for errors occurring, for example, during projection and exposure of a pattern; such a re-sizing of features of the sub-pattern is referred to hereinafter as biasing and/or Optical Proximity Correction (“OPC”). An amount of biasing and/or OPC is also commonly expressed in terms of a corresponding, nominal amount of re-sizing at substrate level. The noun “target” when used in expressions such as “target features” is indicating that these features have substantially a nominal size as desired for the device layer.
Fabrication of a circuit pattern involves the control of space tolerances between features, interconnecting lines, and between elements of a feature as well as the control of the size of features and feature elements. With increasing demands on the number of features per area of die to be printed, resolution enhancement techniques have been developed to improve the resolution limit obtainable with a lithographic processing method using a projection lithography apparatus. The smallest space between two lines permitted in the fabrication of a device layer and/or the smallest width of a line or of any other feature such as, for example, a contact hole, is referred to as the critical dimension (“CD”). Features comprising a minimum size substantially equal to the CD are referred to as “CD-sized features” in the present text.
Optimal performance of a lithographic processing method and usage of the lithographic projection apparatus at its ultimate resolution are specifically required for the lithographic processing of patterns comprising arrays of CD-sized features (such as for example contact holes) spaced apart at a distance substantially equal to the CD. For such arrays of features a pitch P of the periodicity can be defined, which then in this case is substantially equal to twice the CD. A pitch whereby P=2 CD is the minimum pitch at which CD-sized features can be arranged for printing with a lithographic processing method. The layer may also comprise CD-sized features positioned in one or more arrays at a pitches larger than 2 CD. In particular, the printing of layers comprising CD-sized contact holes occurring at both minimum pitch and larger pitches is of importance and requires state-of-the-art resolution enhancement measures. Generally, “dense features” are commonly known to be separated apart by a distance ranging between one and two times the target feature dimension; similarly, “isolated features” are commonly known not to be separated apart by a distance less than two times the target feature dimension. However, there is no commonly accepted exact definition of “dense features;” neither is there a commonly accepted exact definition of “isolated features.” In the text hereinafter, CD-sized features occurring at any pitch between the minimum pitch of 2 CD and a pitch of 3 CD may be referred to as dense features, and CD-sized features occurring at pitches larger than 3 CD may be referred to as isolated features.
Further, the concept of pitch in the present text and claims also applies to clusters of at least two features, in which case “pitch” refers to the mutual distance between two corresponding points of two identical, neighboring features.
The resolution limit of the projection apparatus is one of the characteristics determining the CD obtainable with a lithographic manufacturing process. This resolution limit is generally dictated by the NA of the projection system and the wavelength of the radiation of the projection beam. The conventional approach to enhance resolution is to increase the NA and to reduce the wavelength. These measures have as side effect that depth of focus and insensitivity to residual errors in exposure dose of irradiated target portions become small. The combined usable depth of focus and allowable variance of exposure dose for a given tolerance in the size CD of a CD-sized feature as processed is usually referred to as process latitude. Preferably resolution enhancement measures should not affect process latitude, and therefore a minimum required and obtainable process latitude is presently another characteristic determining the smallest CD obtainable with a lithographic manufacturing process.
Resolution enhancement can be obtained by applying, for example, off-axis illumination modes for imaging dense features. Also, the use of on-axis illumination in combination with a phase shift mask (“PSM”) as a patterning device for imaging isolated and/or dense features is known. For example, an alternating PSM whereby a device pattern is embodied as an electric field phase-shifting pattern of transmissive material with phase shifts of either 180° phase shift or 0° phase shift can be used to print dense line space structures of sub-wavelength pitch. Presently, resolution enhancement is of particular importance for printing device layers comprising both dense and isolated CD-sized contact holes. For printing (i.e., exposing and resist processing) these layers, typically an attenuated Phase Shift Mask (referred to hereinafter as an “att PSM”) is used for patterning the projection beam, and further, the projection system is set at maximum NA and a conventional illumination at high σ-outer setting is used. For example, contact holes of 90 nm size occurring at minimum and larger pitches can in principle be printed with a single exposure lithographic processing method using a projection lithography apparatus equipped with a 193 nm wavelength radiation source and a projection system of NA=0.9, whereby the illumination mode is set at σ-outer=0.75 and a 6% att PSM is used for patterning the projection beam. The process latitude, however, is very critical with respect to depth of focus. For the example described above at 8% exposure latitude only about 110 nm depth of focus is available. Typically, because of substrate unflatness and residual focus errors a depth of focus of the order of 200 to 300 nm is required for enabling control of CD within limits in a manufacturing site; with a depth of focus of the order of 110 nm production of device layers with 90 nm contact holes at minimum and larger pitches is not feasible.
To improve resolution limit so-called “double exposure” processes are used as well. Typically, a lithographic processing method whereby an alternating PSM is used as a patterning device involves two exposure steps. Since only 180° phase shift or 0° phase shift are patterning parameters, alternating PSM's feature, besides desired phase shift transitions, also inevitable undesired phase shift transitions from 180° phase shift to 0° on transparent regions of the mask. The undesired phase shift transitions give rise to undesired intensity dips in an aerial image of the pattern. The latter intensity dips can be compensated for by a second exposure (to level out intensity dips). The two exposures can be performed with corresponding optimally chosen (but generally different) illumination settings. This process, however, does not result in a substantial improvement of process latitude.
Another double exposure approach addresses the problem of the occurrence of spurious features (“side lobes”) in a projected image of a reticle pattern of dense contact holes, due to interference of radiation diffracted at the reticle pattern. To alleviate this problem, the pattern of dense contact holes is split up in two or more partial patterns of less densely packed contact holes. Imaging of such less dense patterns is less critical with respect to side lobes, and leads to an improvement of process latitude, however the improvement is a fractional one. Therefore, there is the problem of enhancing process latitude of a lithographic processing method for printing patterns comprising both isolated and dense features while at least maintaining sufficient the resolution.